The present invention relates to a heat-resistant flexible laminate for substrate of printed circuit board and a method for the preparation thereof. More particularly, the invention relates to a flexible laminate for substrate of printed circuit board composed of a heat-resistant electrically insulating base film and a metal foil integrally bonded to the base film by use of an adhesive, of which the adhesive bonding strength between the base film and the metal foil is greatly improved.
Along with the recent trend toward more and more increased variety and compactness of electronic instruments, the electric circuitry in such an electronic instrument is increasingly provided as an assembly of printed circuit boards each prepared on a flexible laminate as a substrate composed of a base film of an electrically insulating synthetic resin and a metal foil bonded to the base and etched in a pattern of the desired electric circuit by virtue of the merit that such flexible printed circuit boards are very light in weight and can be mounted three-dimensionally in the instrument. It is of course usual that such a printed circuit board is finished by connecting various circuit elements or devices such as resistors, capacitors, coils, ICs, transistors, switch terminals and the like to the circuit pattern by soldering.
As is mentioned above, the flexible laminate for substrate of a printed circuit board is prepared by integrally bonding a base film of a synthetic resin and a metal foil by use of an adhesive. Various kinds of plastic films can be used as the flexible base film including polyethylene terephthalate films and polyimide films, of which the former films are less preferable than the latter films due to the relatively low heat resistance thereof to withstand the heat unavoidable since the circuit elements or devices must be connected to the circuit pattern by soldering. Therefore, most of the flexible foiled laminates for printed circuit boards are prepared with a polyimide film as the base in order to be resistant against heat by soldering.
The adhesive used to adhesively bond the base film of a synthetic resin and the metal foil is naturally required to have flexibility in order to ensure flexibility of the finished circuit board per se and the adhesives currently used for the purpose are exemplified by NBR-based adhesives, polyamide-based adhesives, acrylic rubber-based adhesives and the like. Another requirement for the adhesive is also the heat resistance and even the acrylic rubber-based adhesives, having the highest heat resistance among the above named ones, can hardly withstand a heating condition of 1 minute at 250.degree. C. not to be satisfactory when the mounting works of the circuit elements or devices on to the printed circuit board includes a step of soldering. For example, difficulties are frequently encountered in the repair works of the printed circuit board because of the poor heat resistance of the adhesive not to withstand heating for a sufficient length of time required for the repair works by soldering. Furthermore, the printed circuit boards are not rarely used at an elevated temperature so that high heat resistance is essential even by setting aside the problems in the repair works with soldering.
A possibility of obtaining high heat resistance of a printed circuit board may be provided by use of a silicone- or organopolysiloxane-based adhesive including so-called silicone resin and silicone rubber adhesives known generally to have good heat resistance in comparison with ordinary organic adhesives. A problem in the use of silicone-based adhesives is their relatively low adhesiveness so that a sufficiently high adhesive bonding strength can be obtained only by subjecting the base film to a pretreatment of surface modification such as priming prior to the adhesive bonding of a metal foil thereto. Despite such a troublesome pretreatment, no practically usable flexible laminate for printed circuit boards has yet been obtained.